1. Field of the Invention
The present invention related generally to the use of a zero-valent metal emulsion to remediate contaminants found in a natural resource, such as groundwater, surface water, soils and sediment.
2. Description of Related Art
The removal of contaminants from natural resources is an ongoing and significant problem. Numerous tactics have been considered with various degrees of success.
Remediation of soils and groundwater contaminated with chlorinated hydrocarbons is important because of the potential carcinogenic nature of the parent compounds and the products of degradation, and their resistance to natural attenuation in the environment. For example, conventional wastewater treatment processes are unable to bring down the trichloroethylene (TCE) levels to drinking water standards (5 ppb). Various ex situ processes like activated carbon adsorption and air stripping are effective in achieving drinking water standards. But, with these methods, the contaminant still must be removed from the solid or vapor medium. The use of advanced oxidation processes such as UV/O3/H2O2 has been reported. However, these processes are likewise limited to ex situ treatment.
The remediation of polychlorinated biphenyls (PCBs) and other chlorinated synthetic aromatic compounds are of great concern due to their toxicity and persistence in the environment. PCBs entered the groundwater, surface water, sediment and soil environments through improper disposal and leaks from heat exchangers, transformers, and hydraulic systems. PCBs were used in many industrial applications because of their robust physical and chemical properties such as their resistance to acids, bases and oxidation, their excellent dielectric characteristics and their thermal stability at high temperatures (up to 350° C.). When released into the environment, PCBs are sorbed to particulate matter that can then disperse over large areas. PCBs can be introduced into the food chain by the uptake of contaminated sediments by biota at the water sediment interface. Although the U.S. Environmental Protection Agency (EPA) has banned the manufacture of PCBs since 1979, they are still present in the environment posing possible adverse health affects to both humans and animals. Thus, it is of utmost importance to develop a method that remediates PCBs contaminated soils and waters.
There are presently no widely accepted methods for the remediation of soils or water that are contaminated with PCBs. Several methods have been proposed and tested, both ex-situ and in-situ including the use of microorganisms for the aerobic and anaerobic biodegradation of PCBs in soils, the incineration of PCB laden soils, and other chemical methods like solvent extraction. However, all these methods require long times and/or very elevated temperatures for the dehalogenation to occur and are not practical and usually end up being very expensive.
Another growing environmental concern in the world today is that of contamination of soils and sediments by heavy metal ions such as lead and arsenic. This type of contamination is caused primarily by smelting and mining activities. Heavy metals leave contaminated areas unusable for agricultural and residential purposes. Additionally, heavy metals may pollute drinking water and cause health problems to individuals when the heavy metals leach into the groundwater system. Therefore, strict governmental regulations have been enacted which require businesses associated with contaminated sites to remove the heavy metals from the ground. Although the polluting source may be controlled or eliminated, methods which would provide a safe means of removing the heavy metals entirely from the ground are of great importance. Additionally, any method used to remove the heavy metals from the ground should be environmentally safe. A method that would remove the heavy metals without producing additional hazardous waste would be most advantageous. Current techniques used for remediation of these contaminants are primarily ex situ, which increases the cost and difficulty of such clean up operations. An in situ technique would be much simpler to implement and more cost effective than the techniques in use today.
Several researchers have demonstrated that zero-valent iron is very effective in the treatment of many chlorinated hydrocarbons, such as dissolved trichloroethylene (TCE). Nanometer size iron particles have also been incorporated into a slurry and then injected into a soil matrix to form a permeable reactive barrier (PBR) to intercept a contaminated groundwater plume. All known PRB technologies have been used to treat contaminated groundwater plumes by intercepting them while leaving the source of the contamination untreated. The quality of dense non-aqueous phase liquid (DNAPL) in the source may be at such levels that they could feed and contaminate a plume for hundreds of years, thus requiring that the PRB be active for that many years. Additionally, the DNAPL frontal boundary is hydrophobic and will reject a hydrophilic iron particle slurry, thus preventing the dehalogenation from occurring.
Previous research proved the feasibility of using an emulsified system to dehalogenate a DNAPL source, such as TCE, in the subsurface by means of an in-situ injection. To accomplish this, a dense reactive emulsion was generated that when injected into the ground, would enter the DNAPL pool, encapsulate it and degrade the chlorinated solvents into benign by-products. This emulsified system consisted of a surfactant stabilized oil-in-water emulsion with reactive iron contained within the emulsion micro-droplets. The surfactant served two purposes: it made the emulsion stable for injection into the DNAPL and it aided in the delivery of TCE to the iron. The generation of a hydrophobic emulsion system drew the DNAPL TCE through the oil membrane where it diffused to the iron particle and underwent degradation. TCE continued to enter, diffuse, degrade and exit the micro-droplet maintaining a concentration gradient across the membrane, thus maintaining the driving force of the reaction. This iron-emulsion system can be delivered in-situ to a DNAPL pool through a series of push wells. Research was done to prove that it was possible to introduce an emulsified system by way of slurry injection into a soil matrix. However, zero-valent iron alone is unable to completely dechlorinate PCBs dissolved in aqueous solutions.
Prior research has indicated that emulsified zero-valent iron is a useful technique for the in situ remediation of DNAPLs such as trichloroethylene. An emulsion that is adapted for the in situ removal of heavy metal ions would be desirable. This emulsion would be environmental friendly and cost effective.